The continuously variable hydrostatic transmissions disclosed in the cited copending applications include a hydraulic pump unit and a hydraulic motor unit positioned in opposed, axially aligned relation with an intermediate, wedge-shaped swashplate. The pump unit is connected to an input shaft driven by a prime mover, while the motor unit is grounded to the stationary machine housing. An output shaft, coaxial with the input shaft and drivingly coupled to a load, is pivotally connected to the swashplate in torque-coupled relation. When the pump unit is driven by the prime mover, hydraulic fluid is pumped back and forth between the pump and motor units through ports in the swashplate. As a result, three torque components, all acting in the same direction, are exerted on the swashplate to produce output torque on the output shaft for driving the load. Two of these torque components are a mechanical component exerted on the swashplate by the rotating pump unit and a hydromechanical component exerted on the swashplate by the motor unit. The third component is a pure hydrostatic component resulting from the differential forces created by the fluid pressures acting on circumferentially opposed end surfaces of the swashplate ports, which are of different surface areas due to the wedge shape of the swashplate.
To change transmission ratio, the angular orientation of the swashplate relative to the axis of the output shaft is varied by a ratio controller. Since the transmission ratio, i.e., ratio of input shaft speed to output shaft speed, is continuously variable between 1:0 and 1:1, the prime mover can run at a constant speed set essentially at its most efficient operating point. The availability of a 1:0 (neutral) transmission ratio setting eliminates the need for a clutch. As is disclosed in cited application Ser. No. 08/342,472, the swashplate can be positioned to angular orientations beyond the 1:0 ratio setting to provide limited infinitely variable speed drive in a reverse direction, as well as to angular orientations beyond the 1:1 setting to provide a limited, infinitely variable, overdrive speed range. Significantly, reverse drive is available without need for a reversing gear mechanism.
Unlike conventional, continuously variable hydrostatic transmissions, wherein hydraulic fluid flow rate increases proportionately with increasing transmission ratio such that maximum flow rate occurs at the highest transmission ratio setting, the flow rate in the transmissions disclosed in the cited applications reaches a maximum at a midpoint in the ratio range and then progressively decreases to essentially zero at the 1:1 transmission ratio setting. Thus, losses due to hydraulic fluid flow are reduced, and the annoying whine of conventional hydrostatic transmissions at high ratios is avoided. By virtue of the multiple torque components exerted on the swashplate, the decreasing hydraulic fluid flow in the upper half of the output speed range, and the capability of accommodating a prime mover input operating at or near its optimum performance point, the hydraulic machines of the cited U.S. patent applications have a particularly advantageous application as a highly efficient, quiet, continuously variable hydrostatic transmission in vehicular drive trains.
To accommodate operation at the infinitely variable transmission ratios set by the angular orientations of the swashplate, cylinder blocks of the hydraulic pump and motor units, pressed in sliding, interfacial engagement with opposed faces of the swashplate, are mounted by large spherical bearings concentric with the output shaft axis. These spherical bearings permit independent nutating motions of the pump and motor cylinder blocks during transmission operation; the amplitudes of these nutating motions being determined by the inclination angles of the engaging swashplate faces, which, in turn, are determined by the transmission ratio-setting angular orientation of the swashplate. It is the nutating motions of the pump and motor cylinder blocks that produce the hydraulic fluid pumping action of axially fixed pistons received in cylinders of the cylinder blocks. Since these spherical bearings for the pump and motor cylinder blocks are positioned at opposite sides of the swashplate in axially displaced relation to the pivotal connection of the swashplate to the output shaft, the centers of the cylinder block nutating motions and the center of the swashplate ratio-changing pivotal motion are located at three axially displaced points along the output shaft axis.
An important operating requirement is the achievement of a requisite degree of balance of the hydraulic forces exerted by the cylinder blocks and the swashplate on each other, such that the pump and motor cylinder blocks are continuously maintained in substantially fluid tight, interfacial engagement with the swashplate faces. Factors that complicate achievement of this requirement are the axially displaced centers of nutating motions of the pump and motor cylinder blocks noted above and, except for the transmission ratio setting where the inclination angles of the swashplate faces are equal, the force exerted on the swashplate by the cylinder blocks are unequal. Consequently, there is a resultant moment on the swashplate that must be balanced in order to maintain a desired swashplate angular orientation (set a transmission ratio) and that must be overcome in order to change the swashplate angular orientation (stroke the transmission) through a range of transmission ratios (speed range). Unfortunately, the magnitude of this resultant swashplate moment is variable, depending upon hydraulic pressure, swashplate angle (transmission ratio) and various other geometrical conditions, and achieves substantial magnitudes. The ratio controller must therefore exert sufficient force on the swashplate to reliably set a transmission ratio, as well as to change transmission ratio. Consequently, the ratio controller must be particularly forceful and robust and therefore occupy considerable space within the transmission housing.